Biomechanics of Birth-Related Injuries The overall goal of this research is to better understand the mechanisms of maternal vaginal birthrelated injury at the end of the second stage of labor. The main factor affecting the resistance of the pelvic floor muscles to stretch as they resist the downward descent of the fetal head is their viscoleastic material properties. The effect of term pregnancy on these properties has never been documented, partly due to the difficulty of obtaining sample of undamaged human pelvic floor tissues during birth. In AIM 1, therefore, we will use equi-biaxial testing and stepwise stress relaxation assays to characterize the effect of term pregnancy on the constitutive law and mechanical behavior of mammalian pelvic floor tissues in rat and squirrel monkey. Uniaxial failure tests will also be conducted to determine the effect of pregnancy and test direction on the ultimate tensile stress in these tissues. In AIM 2.1 we will develop a subject-specific, 3-D finite element biomechanical model of the second stage of labor from Station +2 on with representations of the fetal head, five major pelvic floor muscles and related soft tissues, as well as the time-varying maternal expulsive force. In AIM 2.2 we will validate the model predictions by comparing them against the results of in vivo experiments in pregnant women. These involve the measured temporal displacement of a posterior weighted speculum at Csection, and the time course of the increase in vaginal diameter upon fetal head crowning. In AIM 2.3 we will investigate the effect of (a) fetal head orientation, (b) cephalopelvic disproportion, (c) maternal sub-pubic arch angle, (d) epidural, (e) forceps use, and (f) episiotomy on the magnitude, direction and location of maximum pelvic floor muscle tissue stress. The ratio of that stress to the ultimate tensile stress is taken as a measure of the risk of tissue injury. These observations will yield insights into the factors associated with the greatest risk for injury, and should lead to better methods of preventing these injuries.